We show that dc voltage versus current measurements of a YBa(2)Cu(3)O(7-delta) film in a magnetic field can be collapsed onto scaling functions proposed by Fisher et al. [Phys. Rev. B 43, 130 (1991)] as is widely reported in the literature. We find, however, that good data collapse is achieved for a wide range of critical exponents and temperatures. These results strongly suggest that agreement with scaling alone does not prove the existence of a phase transition. We propose a criterion to determine if the data collapse is valid, and thus if a phase transition occurs. To our knowledge, none of the data reported in the literature meet our criterion.

We measure the local harmonic generation from superconducting thin films at microwave frequencies to investigate the intrinsic nonlinear Meissner effect near T c in zero magnetic field. Both second and third harmonic generation are measured to identify time-reversal symmetry breaking (TRSB) and time-reversal symmetric (TRS) nonlinearities. We perform a systematic doping-dependent study of the nonlinear response and find that the TRS characteristic nonlinearity current density scale follows the doping dependence of the de-pairing critical current density. We also extract a spontaneous TRSB characteristic current density scale that onsets at T c , grows with decreasing temperature, and systematically decreases in magnitude (at fixed T/T c ) with under-doping. The origin of this current scale could be Josephson circulating currents or the spontaneous magnetization associated with a TRSB order parameter.

We report on the normal-superconducting phase transition in thick YBa2Cu3O 7−δ films in zero magnetic field. We find significant finite-size effects at low currents even in our thickest films (d = 3200Å). Using data at higher currents, we can unambiguously find Tc and z, and show z = 2.1 ± 0.15, as expected for the three-dimensional XY model with diffusive dynamics. The crossover to two-dimensional behavior, seen by other researchers in thinner films (d ≤ 500Å), obscures the three-dimensional transition in both zero field and the vortex-glass transition in field, leading to incorrect values of Tc (or Tg), ν, and z. The finite-size effects, usually ignored in thick films, are an explanation for the wide range of critical exponents found in the literature.

As a superconductor goes from the normal state into the superconducting state, the voltage vs. current characteristics at low currents change from linear to non-linear. We show theoretically and experimentally that the addition of current noise to non-linear voltage vs. current curves will create ohmic behavior. Ohmic response at low currents for temperatures below the critical temperature Tc mimics the phase transition and leads to incorrect values for Tc and the critical exponents ν and z. The ohmic response occurs at low currents, when the applied current I0 is smaller than the width of the probability distribution σI , and will occur in both the zero-field transition and the vortex-glass transition. Our results indicate that the transition temperature and critical exponents extracted from the conventional scaling analysis are inaccurate if current noise is not filtered out. This is a possible explanation for the wide range of critical exponents found in the literature.

During layer-by-layer homoepitaxial growth, both the Reflection High-Energy Electron Diffraction (RHEED) intensity and the x-ray reflection intensity will oscillate, and each complete oscillation indicates the addition of one monolayer of material. However, it is well documented, but not well understood, that the phase of the RHEED oscillations is not constant and thus the maxima in the RHEED intensity oscillations do not necessarily occur at the completion of a layer. We demonstrate this using simultaneous in situ x-ray reflectivity and RHEED during layer-by-layer growth of SrTiO3. We show that we can control the RHEED oscillation phase by changing the pre-growth substrate annealing conditions, changing the RHEED oscillation phase by nearly 180• . In addition, during growth via pulsed laser deposition, the exponential relaxation times between each laser pulse can be used to determine when a layer is complete, independent of the phase of the RHEED oscillation.PACS numbers: 68.47. Gh,61.05.cf,81.15.Fg Thin-film growth changed dramatically more than three decades ago with the discovery of reflection high-energy electron diffraction (RHEED) intensity oscillations.1-3 During RHEED, a high-energy (≈ 10-30 keV) electron beam is fired at grazing incidence onto a growth surface and the intensity of the reflected beam is recorded. With RHEED, the incident electrons only interact with the topmost layer. During 2D monolayerby-monolayer growth, researchers discovered oscillations in the intensity of the reflected RHEED beam, and that the period of the oscillation corresponded to the addition of exactly one monolayer to the film. This discovery led to rapid implementation of RHEED systems for thinfilm growth, although largely restricted to semiconductor growth via molecular beam epitaxy due to the low pressures (< 10 −5 mbar) required to use RHEED. 4,5The landscape changed again nearly two decades ago with the development of high-pressure RHEED systems, allowing RHEED systems to operate at pressures as high as 1 mbar.6-8 Since this discovery, in situ RHEED characterization has become nearly ubiquitous in thinfilm growth systems, and it has been successfully implemented in a variety of growth techniques in addition to molecular beam epitaxy, such as sputtering 9 and pulsed laser deposition (PLD). 10Researchers have worked to understand RHEED intensity oscillations via a variety of different methods. In principle, the complete picture can only be understood using dynamical diffraction theories, 11,12 which often have to be modified for complicated growth conditions (e.g., including variations in the scattering potential 13 or small terrace sizes 14 ). Before resorting to a full model, it often suffices to describe the RHEED intensity oscillations as the interference between two layers via a kinematic scattering approximation, 15,16 though this model can become more complex when multiple layers are included 17 or other diffraction features such as Kikuchi lines are included.18 The other often-used simplification is the step ...